Various kinds of resin-made packaging materials have heretofore been used for resin-made packaging various food or goods. Such resin-made packaging materials include those making use of, for example, films of thermoplastic resins such as polyolefin, polyester, polystyrene and polyvinyl chloride. Since these resin-made packaging materials are generally insufficient in gas barrier properties such as oxygen gas barrier property and carbon dioxide gas barrier property, however, they are unsatisfactory for applications to especially food packaging materials (packaging materials for meat, fishes and shellfishes, dairy products, pickles, miso, confectionery, tea.cndot.coffee, noodles, rice, etc.), toiletry packaging materials, drug packaging materials, etc. In order to improve the gas barrier properties of such resin-made packaging materials, there have thus been developed composite films in which a gas barrier film composed of an ethylene vinyl alcohol copolymer (EVOH) or polyamide is combined therewith.
However, the films formed of EVOH or polyamide are impaired in gas barrier properties to a great extent under high-temperature and high-humidity conditions. Therefore, the conventional composite films containing these films have been insufficient for packaging materials for food and goods which require a treatment under high-temperature and high-humidity conditions, such as retorting, food and goods which particularly require a long-term storage, and the like.
In recent years, biodegradable polymers, for example, polylactic acid, polysuccinates, polycaprolactone, etc. have attracted attention as plastic materials which scarcely impose burden on the environment. Packaging material making use of these biodegradable polymers are also going to be developed. However, films of these biodegradable polymers are insufficient in gas barrier properties such as oxygen gas barrier property and carbon dioxide gas barrier property. When these films are combined with the conventional EVOH or polyamide film to improve their gas barrier properties, a problem of increasing burden on the environment has arisen.
Recently, the present inventors succeeded in producing films having excellent gas barrier properties from polyglycolic acid. However, only a single layer of the polyglycolic acid film is not always sufficient in, for example, heat sealability, moisture resistance, mechanical strength, profitability, etc.